FIG. 1 shows a typical design of such a HVDC transmission short coupling, in which the two power networks NA and NB are each interconnected by a static converter SRA, SRB and the actual d.c. transmission line, with the HVDC transmission line in the simplest design consisting of a reactance coil L. Preceding each of the static converters on its respective a.c. current side is a static converter transformer whose transformation ratio can be adjusted by distinct steps .DELTA.uA .DELTA.uB in comparison to an average value. In actual situations often several individual converters are connected to the power network on the a.c. side through individual transformers and on the d.c. side connected in series to permit a higher-pulse operation of the respective static converter arrangement. To clarify the invention, however, the discussion will focus exclusively on the control of one static converter.
The valves of the static converters each receive from one control set STA or STB such ignition pulses that in the structure of the control arrangement depicted schematically in FIG. 1 static converter SRA draws a current from power network NA which can be controlled by the control voltage USTA (rectifier operation of static converter SRA in station A). while static converter SRB installed in station B functions as an inverter feeding power network NB, with the supplied power being controlled by its control voltage USTB in such a manner that a d.c. voltage UdB is maintained in the HVDC transmission line which can be measured by means of a voltage sensing device.
To generate the control voltage USTA, a set value Id* is provided by a central control unit STZ for the HVDC current Id measured by the measurement device, with its control differential being compensated by a current controller RIA. The output signal .DELTA..alpha.A of the current controller could basically be directly used as the control voltage for the control set STA. As, however, by means of control voltage USTA primarly only the output voltage of static converter SRA is impressed and the d.c. current fed into the HVDC transmission line--in accordance with the inductive voltage drop at this coil--depends upon the differential of the d.c.-side connecting voltages of static converters SRA and SRB and thus the d.c. voltage UdB prevailing at the d.c.-side connection of static converter SRB functions as a disturbance variable, it is advantageous to connect this d.c. voltage UdB as a pilot control voltage to the controller output signal .DELTA..alpha.A by means of a pilot control device VA in a positive feedback arrangement. Thus, the inductive voltage drop associated with the current set value Id* is eventually impressed by means of control voltage USTA so that the control of the current is independent of any change in UdB. Said type of pilot control is described in U.S. Pat. No. 4,648,018, which also provides for replacing the measured value UdB by a model value which can be calculated by the central control unit STZ using a control and computing element RZ from the current actual value Id, the amplitudes UA and UB of power networks NA and NB, the control angle or the extinction angle .gamma.B of station B and a parameter dx for the inductive voltage drop of the static converters.
Preferably, the pilot control unit VA comprises a linearizing circuit VLA which takes into account that the relationship UdA=UA . cos .alpha.A approximately prevails between the ignition angle .alpha.A of static converter SRA preset by the control voltage and the d.c.-side supply voltage which will be referred to below as UdA. Thus, if ignition angle .alpha.A is preset by the pilot control device VA directly as control voltage in accordance with .alpha.A=USTA=arc cos (.DELTA.A+UdB)/UA), then d.c. voltage UdA=.DELTA..alpha.A+UdB is generated thereby. The controller output signal .DELTA..alpha.A thus determines the voltage differential UdA-UdB and thus permits presetting the voltage drop in the HVDC transmission line as required for impressing the desired current.
The linearization can also be handled differently instead of utilizing an arc cos generator for the sum .DELTA..alpha.A+UdB. Thus, for example. voltage UdB can be converted into a rectifier pilot control angle .alpha.Gv=arc cos (UdB/UA), which then can be corrected by the control output signal .DELTA..alpha.A in accordance with an ignition angle shift .DELTA.A=.alpha.Gv+.DELTA..alpha.A. Below, the pilot control variable supplied to the pilot control device VA is designated as rectifier pilot control angle .alpha.Gv even if it does not yet have the dimension of an angle (e.g., the voltage UdB in FIG. 1), but is only connected within the pilot control device to form the control output signal .DELTA..alpha.A so that the control angle .DELTA.A is the sum of the pilot control angle given by the pilot control variable and an angle correction given by the controller output signal.
With respect to the static converter SRB functioning as an inverter, its d.c. voltage UdB depends upon the inverter control angle .alpha.B as derived from the amplitude UB of power network NB in accordance with the equasion UdB=UB.multidot.cos .alpha.B. The central control and computing element RZ of control unit STZ thus permits control of the HVDC transmission line voltage by presetting the inverter control angle.
When presetting the inverter control angle, attention has to be paid, however, that in the ignition cycle of the static converter valves when igniting a new valve, the current first has to be completely commuted from the valve to be extinguished and this valve to be extinguished must have attained its complete extinguished current level before its valve voltage becomes positive ("inverter threshold limit"). The phase position at which said current commutation is completed is designated as the extinction angle .gamma., and it is thus advantageous for controlling static converter SRB not to preset the ignition angle, but the extinction angle, which can be computed by the control and computing element RZ from preset nominal values for the HVDC power transmission.
Between the extinction angle and the inverter ignition angle to be supplied by the control voltage USTB, there is a phase shift designated as "overlapping angle" u which can be computed in advance, at least approximately, by the control and computing element RZ for each nominal extinction angle .gamma.* in accordance with the instantaneous values of the HVDC transmission current Id and the power network amplitude UB. If then said precalculated overlap angle u along with the output signal .DELTA..alpha.B of a .gamma. controller RGB is converted in a pilot control device VB to generate control voltage USTB, it is possible to regulate the static converter SRB to the desired d.c. voltage by means of an inverter control angle .alpha.B. The pilot control variable determining the overlap angle u for said pilot control device VB is hereinafter designated as .gamma.Wv; a separate linearizing element becomes partially unnecessary in this context and can be replaced in FIG. 1 by an amplifier for level adjustment. Here as well, however, the design of the pilot control device can be largely adjusted to the respective application.
In order that the central control until STZ can generate the set values Id*, .gamma.* and the pilot control signals .alpha.Wv and, if necessary,.alpha.Gv, information regarding amplitudes UA, UB of power networks NA, NB and their properly functioning status is required, which is detected at a power network voltage control device NCT ("network monitor") whose function will be discussed later in greater detail.
FIG. 2 depicts the same HVDC transmission line with its static converters SRA, SRB, their transformers and control sets STA, STB, although here static converter SRA of station A is used as the inverter and static converter SRB of station B as the rectifier.
As may be seen, control set STB of static converter SRB functioning as the rectifier is now controlled by a current controller RIB and pilot-controlled by a pilot control signal .alpha.Gv which is detected either at the output d.c. voltage UdA of rectifier SRA or supplied as a model value by computing element RZ of the central computing network STZ.
The control voltage USTA for control set STA of static converter SRA functioning as inverter is now generated by an extinction-angle controller RGA assigned to station A and pilot-controlled by the inverter pilot control variable.alpha.Wv. The inverter station is supplemented by a marginal current control which was omitted from FIG. 1 for inverter station B for clarity's sake and is not necessarily required for all HVDC transmission networks. Operating conditions can arise in which, due to the control of the power supply to the rectifier station, maintenance of the desired d.c. current is not attained and thus the HVDC transmission line is not fully utilized in terms of its power-transmission capacity.
In said instances it can be advantageous to cease controlling the HVDC transmission current in the inverter station by means of the extinction angle and rather to preset the control voltage USTA in such a fashion that the current supplied into the power network NA is changed by a shift of the inverter ignition angle to assure maintenance of a d.c. nominal value reduced by at least a small fraction ("marginal current" Imarg). For said purpose inverter station A also has a current regulator RIA which is supplied as its nominal value the value of the current regulator RIA and the output signal of the RID, reduced by the marginal current Imarg. The output of this additional current regulator RIA and the output signal of the pilot control device VA are supplied to a selector circuit ASA of station A.
As long as the d.c. current Id remains below the reduced set value Id*-Imarg, the output signal of the current regulator RIA is effected and connected to the control line for USTA in order to increase the d.c. current Id by a change in the control angle. When the current controller RIA changes its polarity or the preset ignition angle defined by the extinction angle controller RGA and its pilot control device VA is attained, then the selector circuit ASA connects the output signal of said extinction angle controller with control line USTA, thereby disconnecting current controller RIA which comes against its stop since its controlled network has now been interrupted. Selector circuit ASA thus assures mutual replacement of controllers RIA and RGA in order to maintain at least the current Id*-Imarg in the HVDC transmission line.
The type of operation of the HVDC transmission line explained thus far based on illustrations 1 and 2 requires, however, in both stations a three-phase network whose phase voltages each have approximately the preset nominal value for proper power network operation, or which are at least symmetrical. A voltage dip or a total collapse of one or two phase voltages produces, however, a non-symmetrical voltage network. While in the event of failure of one phase, the two other phases of a three-phase network could still transmit 66% of the rated power output, the value for d.c. transmission in the HVDC transmission line is substantially lower.
At the d.c. voltage output of the "faulty" static converter which has an operating malfunction, there is now a substantially lower average d.c. voltage. Moreover, the negative phase-sequence network now prevailing in the faulty power network generates transient control angle variants which also reduce the d.c. current. Thus, without any changes in the control parameters, the power output of the HVDC transmission line possible in comparison to the potential power output of the faulty power network is substantially reduced.
In the event of a power network fault on the rectifier side, the voltage dip of the HVDC transmission voltage can produce such a drastic d.c. current change that a total collapse of current flow in the HVDC transmission line results.
Said sudden change of the power output, however, disrupts the stability of the voltage in the non-malfunctioning power network as well.